Our muscles, tendons, and ligaments contribute greatly to spinal alignment, agility, and structure. When not specifically moved (since they cannot be moved by simple, gross actions), the tissues can begin to adhere to each other, lose their resilience, and even tear when suddenly overloaded by resistance or contraction.
Although muscles can repair themselves rapidly, when injured they do contribute greatly to lower back pain. Pinched or cut nerves prevent muscles from functioning, which can occur when a herniated disc presses a nerve.
Furthermore, because our muscles are in continuous communication with our nervous system, fear, anger, nervousness, and so on can contract the injured muscles, leading to muscle spasm. This tension can progress, the muscles can become conditioned, and, as a result, the lower back pain can become chronic.